This invention relates to load pull testing of microwave power transistors, which use automatic microwave tuners in order to synthesize reflection factors (or impedances) at the input and output of the transistors at various harmonic frequencies.
Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient and inaccurate for the transistors operating at high power in their highly non-linear regions and close to saturation, to be described using analytical or numerical models only. Instead the devices must be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing such microwave components (transistors) for high power operation is “load pull” and “source pull”. A typical load pull setup is shown in FIG. 1. It includes a signal source 1, a microwave source tuner 2, a test fixture 3 in which a device under test (DUT) is mounted, an output tuner 4 and a power meter 5. The tuners 2, 4 and the test equipment 1, 5 are remotely controlled 6, 7, 8 by a system computer 9 running appropriate software for tuner control and instrument communication (GPIB) for data collection and processing.
The microwave tuners 2, 4 in the setup of FIG. 1 synthesize the microwave impedances presented to the DUT 3 for which the test takes place. The end-result of a load pull testing session is the dependence of DUT parameters, like Gain, Power and Efficiency etc. on load and source impedance, created by the load and source tuners.
Electro-mechanical tuners [3] have several advantages compared to electronic and active tuners, namely long-term stability, higher handling of microwave power, easier operation and lower cost. Electro-mechanical tuners are made using the slide-screw concept, of which a cross section is shown in FIG. 2, which employs adjustable mechanical obstacles (probes or “slugs”) 10 inserted into the transmission media of the tuners, which is a slotted coaxial or parallel plate airline (slabline) 11, FIG. 2, at an adjustable distance from the central conductor 12; this insertion 13 of the slugs creates capacitive coupling between probe and central conductor and allows to reflect part of the power coming out of the DUT output port back into the DUT and to create, this way, a controllable reflection factor (or impedance) that is presented to the DUT.
When the DUT's (transistors) operate at high power, close to power saturation, they become strongly non-linear and deform the sinusoidal input signal injected by the signal source and therefore, following Fourier's law, they generate power at harmonic frequencies. If the injected signal is at a fundamental frequency fo then harmonic power is generated at 2fo, 3fo, 4fo etc. (FIG. 1). This harmonic power is normally lost when fed into a wideband RF load 5 and reduces the operational efficiency of the transistor 3. A test method, which allows the harmonic power to be recovered and re-injected back into the DUT, is called “harmonic load pull”. Harmonic load pull uses harmonic tuners in order to manipulate the impedances at the harmonic frequencies. A typical harmonic load pull setup is shown in FIG. 3.